反射基础之Constructor

代码拾遗

发布于 2018-07-24 15:55:48

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发布于 2018-07-24 15:55:48

文章被收录于专栏：代码拾遗

查找构造器

构造器的声明包含了：名字，修饰符，参数和异常。可以通过java.lang.reflect.Constructor类获取这些信息。下面的例子描述了如何获取构造器：

import java.lang.reflect.Constructor;
import java.lang.reflect.Type;
import static java.lang.System.out;

public class ConstructorSift {
    public static void main(String... args) {
    try {
        Class<?> cArg = Class.forName(args[1]);

        Class<?> c = Class.forName(args[0]);
        Constructor[] allConstructors = c.getDeclaredConstructors();
        for (Constructor ctor : allConstructors) {
        Class<?>[] pType  = ctor.getParameterTypes();
        for (int i = 0; i < pType.length; i++) {
            if (pType[i].equals(cArg)) {
            out.format("%s%n", ctor.toGenericString());

            Type[] gpType = ctor.getGenericParameterTypes();
            for (int j = 0; j < gpType.length; j++) {
                char ch = (pType[j].equals(cArg) ? '*' : ' ');
                out.format("%7c%s[%d]: %s%n", ch,
                       "GenericParameterType", j, gpType[j]);
            }
            break;
            }
        }
        }

        // production code should handle this exception more gracefully
    } catch (ClassNotFoundException x) {
        x.printStackTrace();
    }
    }
}

getGenericParameterTypes()会从class文件中获取Signature Attribute，如果获取不到将会使用getParameterType()代替。运行：

$ java ConstructorSift java.util.Formatter java.util.Locale
public
java.util.Formatter(java.io.OutputStream,java.lang.String,java.util.Locale)
throws java.io.UnsupportedEncodingException
       GenericParameterType[0]: class java.io.OutputStream
       GenericParameterType[1]: class java.lang.String
      *GenericParameterType[2]: class java.util.Locale
public java.util.Formatter(java.lang.String,java.lang.String,java.util.Locale)
throws java.io.FileNotFoundException,java.io.UnsupportedEncodingException
       GenericParameterType[0]: class java.lang.String
       GenericParameterType[1]: class java.lang.String
      *GenericParameterType[2]: class java.util.Locale
public java.util.Formatter(java.lang.Appendable,java.util.Locale)
       GenericParameterType[0]: interface java.lang.Appendable
      *GenericParameterType[1]: class java.util.Locale
public java.util.Formatter(java.util.Locale)
      *GenericParameterType[0]: class java.util.Locale
public java.util.Formatter(java.io.File,java.lang.String,java.util.Locale)
throws java.io.FileNotFoundException,java.io.UnsupportedEncodingException
       GenericParameterType[0]: class java.io.File
       GenericParameterType[1]: class java.lang.String
      *GenericParameterType[2]: class java.util.Locale
The next example output illustrates how to search for a parameter of type char[] in String.
$ java ConstructorSift java.lang.String "[C"
java.lang.String(int,int,char[])
       GenericParameterType[0]: int
       GenericParameterType[1]: int
      *GenericParameterType[2]: class [C
public java.lang.String(char[],int,int)
      *GenericParameterType[0]: class [C
       GenericParameterType[1]: int
       GenericParameterType[2]: int
public java.lang.String(char[])
      *GenericParameterType[0]: class [C
$ java ConstructorSift java.lang.String "[C"
java.lang.String(int,int,char[])
       GenericParameterType[0]: int
       GenericParameterType[1]: int
      *GenericParameterType[2]: class [C
public java.lang.String(char[],int,int)
      *GenericParameterType[0]: class [C
       GenericParameterType[1]: int
       GenericParameterType[2]: int
public java.lang.String(char[])
      *GenericParameterType[0]: class [C

检索并解析构造器修饰符

构造器的修饰符有：

访问修饰符：public，protected，private
注解

下面的例子显示了如何获取修饰符：

import java.lang.reflect.Constructor;
import java.lang.reflect.Modifier;
import java.lang.reflect.Type;
import static java.lang.System.out;

public class ConstructorAccess {
    public static void main(String... args) {
    try {
        Class<?> c = Class.forName(args[0]);
        Constructor[] allConstructors = c.getDeclaredConstructors();
        for (Constructor ctor : allConstructors) {
        int searchMod = modifierFromString(args[1]);
        int mods = accessModifiers(ctor.getModifiers());
        if (searchMod == mods) {
            out.format("%s%n", ctor.toGenericString());
            out.format("  [ synthetic=%-5b var_args=%-5b ]%n",
                   ctor.isSynthetic(), ctor.isVarArgs());
        }
        }

        // production code should handle this exception more gracefully
    } catch (ClassNotFoundException x) {
        x.printStackTrace();
    }
    }

    private static int accessModifiers(int m) {
    return m & (Modifier.PUBLIC | Modifier.PRIVATE | Modifier.PROTECTED);
    }

    private static int modifierFromString(String s) {
    if ("public".equals(s))               return Modifier.PUBLIC;
    else if ("protected".equals(s))       return Modifier.PROTECTED;
    else if ("private".equals(s))         return Modifier.PRIVATE;
    else if ("package-private".equals(s)) return 0;
    else return -1;
    }
}

运行如下：

There is not an explicit Modifier constant which corresponds to "package-private" access, so it is necessary to check for the absence of all three access modifiers to identify a package-private constructor.
This output shows the private constructors in java.io.File:
$ java ConstructorAccess java.io.File private
private java.io.File(java.lang.String,int)
  [ synthetic=false var_args=false ]
private java.io.File(java.lang.String,java.io.File)
  [ synthetic=false var_args=false ]

创建实例

创建实例的方法有两种，一种使用java.lang.reflect.Constructor.newInstance()创建一种使用Class.newInstance()创建。两种主要的区别是

Class.newInstance() 只能调用无参数的构造器，Constructor.newInstance()可以接收多个参数。
Class.newInstance() 抛出原生异常，Constructor.newInstance()会使用InvocationTargeException包装异常
Class.newInstance() 只能使用public，protecte构造器，Constructor.newInstance()在某些情况下可以调用private构造器。

有时构造完成后需要访问类的内部状态。比如需要获取java.io.Console内部设置的字符集。示例如下：

import java.io.Console;
import java.nio.charset.Charset;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import static java.lang.System.out;

public class ConsoleCharset {
    public static void main(String... args) {
    Constructor[] ctors = Console.class.getDeclaredConstructors();
    Constructor ctor = null;
    for (int i = 0; i < ctors.length; i++) {
        ctor = ctors[i];
        if (ctor.getGenericParameterTypes().length == 0)
        break;
    }

    try {
        ctor.setAccessible(true);
         Console c = (Console)ctor.newInstance();
        Field f = c.getClass().getDeclaredField("cs");
        f.setAccessible(true);
        out.format("Console charset         :  %s%n", f.get(c));
        out.format("Charset.defaultCharset():  %s%n",
               Charset.defaultCharset());

        // production code should handle these exceptions more gracefully
    } catch (InstantiationException x) {
        x.printStackTrace();
     } catch (InvocationTargetException x) {
         x.printStackTrace();
    } catch (IllegalAccessException x) {
        x.printStackTrace();
    } catch (NoSuchFieldException x) {
        x.printStackTrace();
    }
    }
}

运行：

$ java ConsoleCharset
Console charset          :  ISO-8859-1
Charset.defaultCharset() :  ISO-8859-1

Constructor.newInstance()可以使用参数构造实例：

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import static java.lang.System.out;

class EmailAliases {
    private Set<String> aliases;
    private EmailAliases(HashMap<String, String> h) {
    aliases = h.keySet();
    }

    public void printKeys() {
    out.format("Mail keys:%n");
    for (String k : aliases)
        out.format("  %s%n", k);
    }
}

public class RestoreAliases {

    private static Map<String, String> defaultAliases = new HashMap<String, String>();
    static {
    defaultAliases.put("Duke", "duke@i-love-java");
    defaultAliases.put("Fang", "fang@evil-jealous-twin");
    }

    public static void main(String... args) {
    try {
        Constructor ctor = EmailAliases.class.getDeclaredConstructor(HashMap.class);
        ctor.setAccessible(true);
        EmailAliases email = (EmailAliases)ctor.newInstance(defaultAliases);
        email.printKeys();

        // production code should handle these exceptions more gracefully
    } catch (InstantiationException x) {
        x.printStackTrace();
    } catch (IllegalAccessException x) {
        x.printStackTrace();
    } catch (InvocationTargetException x) {
        x.printStackTrace();
    } catch (NoSuchMethodException x) {
        x.printStackTrace();
    }
    }
}

运行如下：

$ java RestoreAliases
Mail keys:
  Duke
  Fang

问题排查

由于没有无参数构造器抛出：InstantiationException异常

示例如下：

public class ConstructorTrouble {
    private ConstructorTrouble(int i) {}

    public static void main(String... args){
    try {
        Class<?> c = Class.forName("ConstructorTrouble");
        Object o = c.newInstance();  // InstantiationException

        // production code should handle these exceptions more gracefully
    } catch (ClassNotFoundException x) {
        x.printStackTrace();
    } catch (InstantiationException x) {
        x.printStackTrace();
    } catch (IllegalAccessException x) {
        x.printStackTrace();
    }
    }
}

运行结果：

$ java ConstructorTrouble
java.lang.InstantiationException: ConstructorTrouble
        at java.lang.Class.newInstance0(Class.java:340)
        at java.lang.Class.newInstance(Class.java:308)
        at ConstructorTrouble.main(ConstructorTrouble.java:7)

Class.newInstance() 抛出无法处理的异常

因为Class.newInstance() 并不会包装异常，所以无论是checked还是unchecked的异常都会直接抛出，对于Constructor.newInstance()则会通过InvocationTargetException包装抛出的异常。

import java.lang.reflect.InvocationTargetException;
import static java.lang.System.err;

public class ConstructorTroubleToo {
    public ConstructorTroubleToo() {
     throw new RuntimeException("exception in constructor");
    }

    public static void main(String... args) {
    try {
        Class<?> c = Class.forName("ConstructorTroubleToo");
        // Method propagetes any exception thrown by the constructor
        // (including checked exceptions).
        if (args.length > 0 && args[0].equals("class")) {
        Object o = c.newInstance();
        } else {
        Object o = c.getConstructor().newInstance();
        }

        // production code should handle these exceptions more gracefully
    } catch (ClassNotFoundException x) {
        x.printStackTrace();
    } catch (InstantiationException x) {
        x.printStackTrace();
    } catch (IllegalAccessException x) {
        x.printStackTrace();
    } catch (NoSuchMethodException x) {
        x.printStackTrace();
    } catch (InvocationTargetException x) {
        x.printStackTrace();
        err.format("%n%nCaught exception: %s%n", x.getCause());
    }
    }
}

运行结果：

$ java ConstructorTroubleToo class
Exception in thread "main" java.lang.RuntimeException: exception in constructor
        at ConstructorTroubleToo.<init>(ConstructorTroubleToo.java:6)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance
          (NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance
          (DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at java.lang.Class.newInstance0(Class.java:355)
        at java.lang.Class.newInstance(Class.java:308)
        at ConstructorTroubleToo.main(ConstructorTroubleToo.java:15)
$ java ConstructorTroubleToo
java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance
          (NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance
          (DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at ConstructorTroubleToo.main(ConstructorTroubleToo.java:17)
Caused by: java.lang.RuntimeException: exception in constructor
        at ConstructorTroubleToo.<init>(ConstructorTroubleToo.java:6)
        ... 5 more

执行错误的构造函数

示例如下：

import java.lang.reflect.InvocationTargetException;
import static java.lang.System.out;

public class ConstructorTroubleAgain {
    public ConstructorTroubleAgain() {}

    public ConstructorTroubleAgain(Integer i) {}

    public ConstructorTroubleAgain(Object o) {
    out.format("Constructor passed Object%n");
    }

    public ConstructorTroubleAgain(String s) {
    out.format("Constructor passed String%n");
    }

    public static void main(String... args){
    String argType = (args.length == 0 ? "" : args[0]);
    try {
        Class<?> c = Class.forName("ConstructorTroubleAgain");
        if ("".equals(argType)) {
        // IllegalArgumentException: wrong number of arguments
        Object o = c.getConstructor().newInstance("foo");
        } else if ("int".equals(argType)) {
        // NoSuchMethodException - looking for int, have Integer
        Object o = c.getConstructor(int.class);
        } else if ("Object".equals(argType)) {
        // newInstance() does not perform method resolution
        Object o = c.getConstructor(Object.class).newInstance("foo");
        } else {
        assert false;
        }

        // production code should handle these exceptions more gracefully
    } catch (ClassNotFoundException x) {
        x.printStackTrace();
    } catch (NoSuchMethodException x) {
        x.printStackTrace();
    } catch (InvocationTargetException x) {
        x.printStackTrace();
    } catch (InstantiationException x) {
        x.printStackTrace();
    } catch (IllegalAccessException x) {
        x.printStackTrace();
    }
    }
}

运行：

$ java ConstructorTroubleAgain
Exception in thread "main" java.lang.IllegalArgumentException: wrong number of
  arguments
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance
          (NativeConstructorAccessorImpl.java:39)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance
          (DelegatingConstructorAccessorImpl.java:27)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
        at ConstructorTroubleAgain.main(ConstructorTroubleAgain.java:23)
$ java ConstructorTroubleAgain int
java.lang.NoSuchMethodException: ConstructorTroubleAgain.<init>(int)
        at java.lang.Class.getConstructor0(Class.java:2706)
        at java.lang.Class.getConstructor(Class.java:1657)
        at ConstructorTroubleAgain.main(ConstructorTroubleAgain.java:26)
$ java ConstructorTroubleAgain Object
Constructor passed Object

第一个异常是因为没有无参构造器，第二个异常是因为反射调用不会尽心自动的装箱和拆箱。

调用不能访问的构造器抛出 IllegalAccessException

示例：

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

class Deny {
    private Deny() {
    System.out.format("Deny constructor%n");
    }
}

public class ConstructorTroubleAccess {
    public static void main(String... args) {
    try {
        Constructor c = Deny.class.getDeclaredConstructor();
//          c.setAccessible(true);   // solution
        c.newInstance();

        // production code should handle these exceptions more gracefully
    } catch (InvocationTargetException x) {
        x.printStackTrace();
    } catch (NoSuchMethodException x) {
        x.printStackTrace();
    } catch (InstantiationException x) {
        x.printStackTrace();
    } catch (IllegalAccessException x) {
        x.printStackTrace();
    }
    }
}

运行结果：

$ java ConstructorTroubleAccess
java.lang.IllegalAccessException: Class ConstructorTroubleAccess can not access
  a member of class Deny with modifiers "private"
        at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:505)
        at ConstructorTroubleAccess.main(ConstructorTroubleAccess.java:15)

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原始发表：2018-05-03，如有侵权请联系 cloudcommunity@tencent.com 删除

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